Sunlight is an environmental risk factor for skin cancer. A great deal of work has been done to characterize and quantify sun exposure. However, sun exposure risk estimates for melanoma, squamous cell carcinoma and basal cell carcinoma vary widely. As the magnitude and dose of sun exposure are difficult to measure directly, majority of studies ask study participants detailed questions about sun exposure history. Almost all the questions rely on recalling low morbidity events that may have occurred several years ago. Hence, these measures are subject to systematic recall bias and random misclassification. Also, mainly case-control study designs have been used thus far, particularly for studies that have evaluated risk for melanoma. Work suggested in this proposal will significantly improve our understanding of skin cancer epidemiology. We have a unique opportunity to test our hypotheses in the environment at the Channing Laboratory where existing longitudinal datasets are based such as the Nurses'Health Study (NHS). Data on exposure to skin cancer-related risk factors, confirmed skin cancer cases and geocoded (i.e. mapped) addresses has already been gathered for NHS. The mentors for this proposal are experienced investigators in dermatology and epidemiology and are familiar with the NHS cohort. They also have considerable experience with the epidemiology of skin cancer and participate in projects that are part of the SPORE for Skin Cancer. Combined with this research environment, experienced mentors and training plan suggested in this proposal, we are in a unique position to accomplish the following specific aims: 1. To examine the relationship between history of sun exposure and basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and malignant melanoma (MM) in NHS. 2. To evaluate the relationship between exposure to ultraviolet radiation (UV) flux in the US (via Robertson Berger meters) and BCC, SCC and MM development (geocoded) in NHS. 3. To assess the relationship between history of sun exposure (Aim 1) and UV radiation (Aim 2) and multiple primary BCCs, SCCs and MMs in NHS.